The present invention relates generally to high voltage current limiting devices and in particular, to high voltage, high current sensor/isolator connected in parallel with a current limiter, electrically isolated by a switching device (a spark gap).
High voltage current limiting fuses have been in service for over half a century. They limit peak value of the fault current when operating in their current limiting mode. It is desirable to keep this peak value of the let-through current as low as possible for any available current. The peak let-through current by a fuse increases with its rated continuous current. Thus, for a fixed maximum available fault current, typically 50 kA, a current limiting fuse with a high rated continuous current (1000 A or greater) may not limit the peak value of the fault current and may not provide the necessary protection. The market demands high current rated fuses with low let-through peak current and energy.
Polymer current limiting devices have been applied to limit current at low voltages, i.e., &lt;660 V, in restricted applications. However, there appears to be no application of these devices at high voltages [1000 V and higher] for over-current protection.
The need for high voltage, high continuous rated current fuses with low peak let-through current capability is on the rise. The art of paralleling existing silver sand technology fuses become saturated at this level since the current limiting range is outside the maximum interrupting current, typically 50 kA. High continuous current rated devices currently available in the Market [U.S. Pat. No. 4,692,577 & U.S. Pat. No. 4,479,105], carry the load current on copper conductors, in parallel with current limiting devices isolated from them. When a system fault occurs, the high fault current is shunted to the current limiting device to work in the current limiting range. These devices need a special circuit to measure the current at all times. Tiny failure in the measuring system, these devices will not interrupt and isolate the faulty circuit.